I'm tempted to answer: with much difficulty, in a highly qualitative way, and only by reading a fair-sized book. There are many decent pop-sci books on string theory; I can't remember the names of any I read, but I'm sure someone can recommend one or two.
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NoldorinNov 2 '10 at 19:36

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I think it's safe to say that a person won't really get anything useful out of such an explanation if he or she doesn't have some background in (quantum) field theory.
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j.c.Nov 2 '10 at 20:28

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Particles we currently consider "point particles" (electrons, quarks, photons, etc.) are actually tiny pieces of string with each a characteristic vibration. They interact in a sort of harmony that results in/manifests as the physical laws we observe.

If anyone with more knowledge in the field can correct me, I ask for improvements. This is just how I personally explain it to people who ask, and I'd hate to give out false information.

actually even QFT already does not consider matter as point particles but as fields (for details see e.g. here)
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Tobias KienzlerMar 10 '11 at 9:30

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@Tobias: the modern definition of point particle is a field with renormlizable interaction and simple short-distance propagator.
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Ron MaimonOct 22 '11 at 6:42

Dear Justin L. Where have you learnt String Theory? I am just curious. During a Msc degree? Your profile still says you are an undergrad. I don`t know how are university degrees in other countries anyway.
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Eduardo Guerras ValeraNov 17 '12 at 17:51

...But if you were in my country, it would be nearly impossible to have had Strings (and diff geometry, GR, QM, QFT and so on, before) in a simple physics degree, here they are simply too short.
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Eduardo Guerras ValeraNov 17 '12 at 18:03

If you have learnt alone, I just would like to know what material have you followed (Peskin & Schroeder for QFT and then Polchinski double book from the beginning... I just want to know)
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Eduardo Guerras ValeraNov 17 '12 at 18:18

String theory is the theory of gravity which starts out by postulating that only things at the boundary of space-time make sense. The local space and time have to be reconstructed from the boundary description. This is called the holographic principle. In the 1960s, a primitive vesion of this idea was called the S-matrix principle.

There are two kinds of boundaries, those far away from everything, called "cosmological" and those which are on top of matter, called "black hole horizons". The description of these two horizons are similar. The oscillations of these boundaries describe the entire space-time nearby.

The detailed form of string theory begins with the postulate that there are black holes that make extended, light, 1 dimensional lines. The vibrations of these black holes then must account for all the particles in the theory, because the vibrations of a black hole encodes anything that can fall through.

@Eduardo: It's heuristic, so it has something to do with it, but it's only rigorously understood in AdS spaces and for certain black holes in certain limits, so the general correspondence between boundary states and interior states is not something one can pontificate about in general. The FLRW horizon is growing with time, that makes it very difficult to understand the state space of quantum gravity on this background, or even whether the universe is a pure state or mixed state, and what the Hilbert space is supposed to be. Realistic cosmologies are unfortunately an open question in ST.
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Ron MaimonNov 17 '12 at 16:18

Greene's book has already been mentioned, but Nova's The Elegant Universe is a somewhat informative 3-hour video series. Obviously, to truly understand the theory's progress, years of mathematics and physics must be studied, preferably at a university.

Some of the leaders in the field are Brian Greene and Michio Kaku. Both have made some Sci channel or Nova series that seemed appropriate for non-physicists.
I haven't been following the latest in physics for a while, but I thought that M-Theory had supplanted string theory some time ago. If so, then Kaku would be the guy. Greene's series and book "The Elegant Universe" also goes into some Quantum mechanics, which might help as well.
These are really the only two players I know of that simplify string theory. Does anyone else have some ideas?

Those are certainly the two most well-known string theorists, though it could be argued that physicists such as Edward Witten are equally important in the field.
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NoldorinNov 2 '10 at 19:40

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Witten is much more important to the field than Greene or Kaku.
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j.c.Nov 2 '10 at 20:30

@j.c.: Yes, but we save his credit for M-Theory. ;)
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VorticoNov 3 '10 at 0:39

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I don't think Witten is known for his communication with non-physicists, though. (Maybe he's good at it, I don't know) But I happen to like The Elegant Universe as a popular description of string theory so +1 for that.
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David Z♦Nov 3 '10 at 22:55